PROJECT SUMMARY A functional immune system is central to human health and, at the same time, alterations of normal lymphoid developmental programs can lead to immune deficiencies, autoimmune diseases and a variety of lymphomas. Germinal centers (GCs) are microanatomical structures formed in secondary lymphoid organs during antigen- stimulated immune responses. GCs are the sites of B cell maturation through clonal expansion, somatic hypermutation, and affinity-mediated selection -- and therefore play a critical role in differentiation of antibody secreting cells and memory B cells. OCA-B is a unique transcriptional coactivator that is highly expressed in GC B cells and GC-derived lymphomas such as diffuse large B cell lymphoma and Burkitt?s lymphoma. In vivo studies have demonstrated that OCA-B is essential for both antigen-dependent B cell differentiation (including GC formation) and normal expression of secondary immunoglobulin genes. However, little is known about the exact role of OCA-B in normal GC formation and GC-derived lymphoma progression. In addition, the mechanism by which OCA-B regulates its target genes is also unclear. In addressing these issues, our preliminary studies have demonstrated (i) a functional occupancy of OCA-B on distal enhancers, in particular the large locus control region (LCR) of the BCL6 gene that encodes a master regulator of GC formation; (ii) direct interactions of OCA- B with the MED1 subunit of the Mediator coactivator complex and with a GC-specific transcription factor (MEF2B) important for BCL6 expression and (iii) a B cell-intrinsic Oca-B dependency for GC B cell differentiation in vivo. Based on our previous OCA-B studies and recent preliminary data, we hypothesize that a predominant role of OCA-B in GC formation involves activation of BCL6 through its distal enhancer region by recruiting and concentrating transcription factors and cofactors, including chromatin/epigenetic factors, that facilitate enhancer- promoter interactions. We also hypothesis that OCA-B plays a critical role in GC-derived lymphomagenesis. To address these issues, we will investigate (i) the function and mechanism of action of OCA-B, through interactions with MED1/Mediator and MEF2B, on the BCL6 LCR/super-enhancer in both ex vivo GC B and lymphoma cells, (ii) the mechanism of action of OCA-B-dependent transcriptional coactivation through rigorous biochemical in vitro transcription assays; (iii) the mechanism of action of OCA-B in GC B cell differentiation in vivo, including identification of key target genes and regulatory networks; (iv) the role of OCA-B in B cell lymphoma progression in vivo. We will use complementary biochemical, genomic, bioinformatic, genetic, gene-editing, cell-based and in vivo (mouse model) approaches. Completion of these aims will advance our understanding of the molecular mechanism of action of OCA-B in GC-specific transcriptional regulation, GC B cell differentiation, and GC- derived B cell lymphomagenesis. Notably, it also will provide clues to new therapeutic strategies for treating B cell lymphomas.